This invention relates generally to electrophotographic reproduction apparatus, such as printers and copiers, which employ pressure for fusing a toned image onto a transfer medium. The invention more particularly concerns such apparatus in which, at least some of the time, images are fused to multiple ply transfer media such as envelopes.
In the case of electrophotographic printers, for example, envelopes as well as cut sheets of paper can serve as the transfer media upon which information is printed. Such a printer typically employs a fuser which fuses a toner image of the information to be printed onto the paper or envelope. This fuser is usually made up of a fuser roller urged against a backup roller to form a pressure nip through which the transfer medium passes. In one form of fuser, the fuser roll is heated and has a relatively rigid outer surface, while the backup roll is softer and can be deformed somewhat to form the pressure nip.
Normally, paper can be fed through this type of fuser without difficulty but envelopes tend to crease, fold, or form wrinkles (known as "treeing").
The cause for these problems seems to be non-uniform feeding across the envelope surface due to the multiple ply nature of the envelope. Efforts to correct this problem have included the use of a segmented pressure roller and the employment of roller designs of different shapes. Another approach to solving the problem is to take advantage of the fact that there typically are non-printing zones at the leading and trailing edges of an envelope. In this approach, a reduced amount of pressure at the fuser nip is applied to the leading and trailing edges of the envelope so that the large force at the nip required for fusing is only applied where toner images will be placed on the envelope. This improves the situation somewhat, but creasing and treeing can still occur along the printable zone of an envelope.
It is the general aim of the invention to provide an electrophotographic reproduction apparatus of the foregoing type in which creasing, folding, and treeing of envelopes is substantially eliminated.